Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds valuable behavioral context: 'errors: on invalid, missing, or malformed input this tool never raises a protocol error — it returns {"error": "<what is wrong and how to fix it>"}' and 'Every call is read-only and idempotent, so after correcting the input it is always safe to retry.' This provides error handling patterns and retry safety beyond the annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a paragraph of several sentences, front-loaded with the main purpose and a concrete example. It includes the extraneous 'FREE' but is otherwise efficient. The error handling and differentiation are integrated naturally. Slightly more brevity could be achieved, but it's well-structured and informative.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's complexity (4 params, 1 required, no nested objects) and that an output schema exists (so return values need not be explained), the description is complete. It covers purpose, usage, error handling, default behavior, and distinguishes from siblings. The example output provides clarity. All necessary information for an agent to correctly select and invoke the tool is present.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 100% coverage, so the baseline is 3. The description adds meaning by stating 'Defaults to the 50/30/20 rule, fully customizable' and providing a typical input/output example. This clarifies the default behavior and shows how the parameters relate to the output, adding value beyond the schema's field descriptions.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's function: 'Split monthly income into needs/wants/savings buckets with weekly equivalents.' It specifies the default 50/30/20 rule and explicitly distinguishes from the sibling 'savings_goal' by stating 'Not for how long a target takes to reach (savings_goal).' This provides a specific verb-resource combination and differentiates from alternatives.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explicitly says 'Use when monthly income needs allocating across buckets. Not for how long a target takes to reach (savings_goal).' It names the alternative tool (savings_goal) and provides guidance on error handling: 'on invalid input this tool never raises a protocol error — it returns an error object... safe to retry.' This clearly tells when and when not to use the tool.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.